1. Field of the Invention
The present invention relates to integrated circuits with two-dimensional image sensors and charge transfer devices for transferring signals read out from the image sensors, and in particular to such a circuit for undertaking a relatively slow readout of the image sensor.
2. Description of the Prior Art
An integrated circuit with a two-dimensional semiconductor image sensor having a plurality of sensor elements arranged in rows and columns, with sensor elements in a particular column being connected by a common column line and in which a barrier gate, a storage capacitor, and a transfer gate, all supplied with respective clock pulse voltages, are disposed between the column lines and various stages of an analog shift register is known from German No. OS 26 42 166 and German No. LP 26 11 771. Incomplete charge transfer can occur in this known circuit as a result of the time span for the readout of the sensor signals through the column lines and the barrier gate into the stages of the shift register being necessarily dimensioned too small. The charges remaining behind as a result of the incomplete charge transfer influence the succeeding charge transfers upon readout of the next lines of the image sensor. This problem is particularly acute when, for example, the circuit is used in the environment of a television display wherein the standard period for scanning and readout of an image line is 64 .mu.s. Moreover, within the 64 .mu.s period, 12 .mu.s are allocated to the blanking gap between two line signals so that the parallel transfer of the signals from a sensor line over the barrier element into the shift register must occur within these 12 .mu.s.
A proposed solution to accelerate the readout from the sensor elements is found in German No. OS 26 42 166 wherein a storage capacitor is inserted between the barrier element and each stage of the shift register into which a sensor signal is entered, with the storage capacitor being connected to the input of the shift register stage via a transfer gate.
Another problem in obtaining an accurate readout from sensors of this type is that of noise suppression, that is, suppression of charges which enter the charge transfer devices which were not directly generated as a result of incident radiation on the sensor elements. Such spurious charges may result from non-uniform characteristics of the sensor elements, imperfect coupling of the clock pulse voltages to the various elements and gates, temperature variations, and the like. One solution to this problem in the context of a two-dimensional image sensor is known from German No. LP 26 11 771 in which two shift registers are arranged next to each other and in which a first readout operation of the column lines is undertaken for readout of the total sensor signal, that is, the signal including the noise, into the first shift register, which is then followed by a second readout operation which ensues after a very short integration period during which substantially no charges are formed as a result of incident radiation so that the second readout represents a so-called fixed pattern noise, or zero signal. A difference circuit at the output of the shift registers receiving the respective signals therefrom determines the difference in the signals and generates as a circuit output a signal representing essentially only the result of the radiation incident on the sensors.
Sensor elements useable in conventional devices as well as in the circuit disclosed herein are known, for example, from the test "Solid State Imaging", P. G. Jespers et al (Editor), Noordhof Int. Publishing, Leyden, The Netherlands and the article "A 1,024 Element Linear CCD Sensor With a New Photodiode Structure", S. Ohba et al, Proceedings IEDM 1977, Washington, at pages 538-541.